use warnings;

use Data::Dumper;

sub mysub {
        warn Dumper \@_;  # globals still set
        die 'single word expected' if (my $name = shift) !~ /^\w+$/;
        warn Dumper \@_;  # you "lost" the globals (due to matching /^\w+$/)
        die 'single word expected' if (my $value = shift) !~ /^\w+$/;
}
$str = "abc def";
$str =~ /(\w+)\s+(\w+)/;
mysub($1, $2);

__END__

$VAR1 = [
          'abc',
          'def'
        ];
$VAR1 = [
          undef
        ];
Use of uninitialized value $value in pattern match (m//) at ./t.pl line 12.
single word expected at ./t.pl line 12.

##</code><code>##

my ($name, $value) = ($str =~ /(\w+)\s+(\w+)/);
mysub($name, $value);